Electron beam exposure mask and method of manufacturing the same and electron beam exposure method

ABSTRACT

In an exposure mask of the present invention, a plurality of opening regions are disposed via crossbeams, each having a size not to be resolved, along peripheral edges of island-like patterns and peninsula-like patterns for shielding transmission of charged particles.

This application is a C-I-P of Ser. No. 08/408,818 filed Mar. 23, 1995,abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electron beam exposure mask and amethod of manufacturing the same, and an electron beam exposure method.

2. Description of the Prior Art

Recently, an integration density of semiconductor integrated circuits(IC) have been improved more and more, and functions of the ICs havebeen increased. Thus, in the fields of industry such as computers,communications, and mechanical control where the ICs are employed,progress of technology has been expected widely. There exists some ICssuch as DRAMS wherein a fourfold increase in integration density hasbeen achieved in the past two or three years. Such high integration canbe attained on the basis of the progress in fine pattern technology.

In an electron beam exposure technology, fine patterns of less than 0.05μm, can be obtained if an alignment accuracy of an exposure mask can beachieved within less than 0.02 μm. But, in such case, it has beenconsidered that such fine patterns cannot be employed in mass productionof the LSI because of its low throughput. However, in recent years, thethroughput of about two sheets per hour has been realized by using ablock exposure scheme or a blanking aperture array (BAA) scheme.

In such cases, a superfine pattern formation scheme, wherein both apattern width and a pattern distance can be formed to be less than 0.20μm, is required for lithography technology used for manufacturing thesemiconductor devices.

In case a resist is exposed to form such superfine patterns, a positivetype resist is often advantageous to a negative type resist. In thenegative type resist, a crosslinking reaction is caused in the resistmaterial by an irradiation of the electron beam. Since the negative typeresist becomes inflated by absorbing a developer in developing processto thus make the superfine patterns swell by the development process,dimensional accuracy of the superfine patterns is not assured. On thecontrary, in the positive type resist, the crosslinked resist materialis cut off by the electron beam or the energy beam. Since the positivetype resist in the region where the electron beam is irradiated ismelted down, the dimensional accuracy of the superfine patterns isreadily assured.

Even if patterns of gate electrodes and capacitor electrodes of thetransistor are exposed by employing a so-called "block exposure" schemewherein the electron beam is shaped by means of a transmission maskformed of silicon, the dimensional accuracy of the gate electrodes canbe easily obtained and surface areas of the capacitor electrodes can beformed widely, when they are exposed on the positive type resist. Sincea capacitor is used as a charge storage capacitor of a memory device,for example, a large size capacitor is preferable.

In case the patterns of the gate electrode and the capacitor are formedin the block exposure transmission mask (also referred to as "blockmask" hereinafter) used for exposing the positive type resist, thecapacitor must be patterned to be surrounded by an opening portion 101formed in a shape of a ring, as shown in FIG. 1A. Also, the gateelectrode of the transistor must be patterned to be surrounded by anopening portion 102 formed like a U-shape, as shown in FIG. 1B. Now thepattern 101a surrounded by the ring opening portion 101 is referred toas an island-like pattern hereinafter, and the pattern 102a surroundedby the U-shaped opening is also referred to as a peninsula-like patternor a tongue-like pattern hereinafter.

However, since the island-like pattern 101a surrounded by the ringopening portion 101 is formed in the air as it is, it is not ofpractical use. In addition, the peninsula-like pattern 102a surroundedby the U-shaped opening portion 102 has small mechanical strength.

Therefore, the island-like pattern 101a and the peninsula-like pattern102a are supported by narrow crossbeams (referred to as bridgingportions hereinafter) which can not shield the exposure electron beam,or otherwise, as shown in FIGS. 2A and 2B, the resist is exposed with 2or more shots using plural masks 103 and 104, each composed of aplurality of divided opening patterns. Such a technique is disclosed inUnexamined Patent Publication (KOKAI) 59-222840 (EP Patent ApplicationNo. 83105177.6 filed on May 25, 1983), for example. However, it isapparent that, if this technique is employed, the throughput isinevitably decreased to about half or less.

Further, when lattice-like meshes formed by a narrow wire not exposed isused, some of the meshes can be covered by a thin film so as not totransmit the electron beam. As a result, the exposure mask having theisland-like patterns and the peninsula-like patterns thereon can beformed in the exposure mask.

Furthermore, the exposure mask used for forming the patterns by using adiffraction effect of light is disclosed in Patent ApplicationPublication (KOKOKU) 61-34667. In this exposure mask, as shown in FIG.3A, "a plurality of small holes 105 are so arranged thickly that lightwaves diffracted by adjacent small holes 105 are overlapped each otheron the resist" to thus form a desired pattern.

According to the above exposure mask using the meshes and the aboveexposure mask using the plurality of small holes to form the desiredpattern, one desired pattern is formed by the plural opening portions,and the lights are diffracted into rear sides of the crossbeams dividingthese opening portions. Thereby, the patterns corresponding to thecrossbeams are not resolved substantially on the resist which is formedon the wafer.

If the exposure mask is used to pass or transmit the light such asultraviolet rays and X-rays, no significant problems occur. However, ifan ionizing radiation such as the electron beams with high energy isirradiated onto the exposure mask, it causes the exposure mask to have ahigh temperature so that the exposure mask will be melted or elongated.

In particular, in order to improve the throughput of the electron beamsexposure scheme, if an irradiation time is shortened by increasing theelectron beam current density up to 40 A/cm² or more, or otherwise ifthe electron beam accelerated more than 40 kV or more is employed, theabove meshes cannot be employed as the exposure mask for the chargedparticles. In general, as the exposure mask for the charged particlessuch as the electron beam, it has been proposed that the silicon plateis used as the material and that hole portions are opened by anisotropicetching and trench etching technique. Since the silicon plate is morestiff than both a thin metal plate and a crystal structure, the exposuremask made of silicon is not melted by the energy beam.

When, using the exposure mask made of the silicon substrate, theisland-like patterns and the peninsula-like patterns described above areexposed, the following problems can occur:

(1) Since heat is accumulated in non-opening regions (energy beamshielding regions) in the exposure mask, the heats must be radiatedeffectively. If the heat is accumulated in the exposure mask, thedimensional accuracy of the exposure mask is deteriorated due to thermalexpansion, or destruction of the exposure mask is caused due to stressgenerated by the thermal expansion.

(2) The island-like patterns and the peninsula-like patterns in theexposure mask must have sufficient mechanical strength. Therefore, theframe portions dividing the exposure mask into the plural openingregions have to be formed widely. However, the crossbeam portions areresolved as the patterns on the resist if they are formed too widely, sothat object of the frame portions cannot be attained.

(3) A layout of the crossbeam portions must be so designed that apattern shape to be exposed can be formed precisely. In other words, asdescribed above, after the exposure mask having the above meshes orsmall holes is exposed by the charged particles (electron beam), it hasnot been apparent how to obtain the resist patterns with high accuracy.For example, in the pattern wherein a plurality of regions divided bythe meshes are covered selectively by the thin film, it is hard toattain the pattern shapes with high accuracy.

Moreover, if, as shown in FIG. 3A, the plurality of holes are disposedmerely in vertical and horizontal directions, the pattern 106 having ajagged edge is formed as shown in FIG. 3B, so that the precise patterncannot be formed.

In addition, charged particles irradiated through the opening regions ofthe exposure mask are distributed unequally on the resist when they passor transmit near the edges of the opening regions. Thus this causes thedegraded pattern accuracy. Note that this phenomenon is not restrictedto the opening regions surrounding the island-like patterns and thepeninsula-like patterns.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electron beamexposure mask used for forming resist patterns with high accuracy and amethod of manufacturing the same, and an electron beam exposure method.

According to an aspect of the present invention, a plurality of openingregions which are aligned along a peripheral edge of a charged particleshielding pattern bridging portions (crossbeam), each having anunresolved size, are formed on the electron beam exposure mask.

In this exposure mask, patterns of bridging portions disposed betweenthe opening regions are not resolved on the resist and are thuseliminated when they are exposed. Therefore, when the island-likepatterns and the peninsula-like patterns for shielding the electron beamare formed on the exposure mask, strength of the exposure mask can beincreased. In addition, since patterns disposed within one electron beamshot area can be formed at a time by one shot of the electron beam, athroughput of the exposure is not lowered.

Also, since a bulge of the opening portion can be easily estimated bythe proximity effect caused by an isolated opening region, the patternaccuracy can be improved by forming the opening pattern narrowly by adimension of the bulge. Moreover, even if a plurality of opening regionsare aligned in plural directions, corners of the patterns surrounded bythe opening regions can be prevented from being rounded off since theopening regions disposed in intersecting regions are formed smaller insize than those of other opening regions.

According to another aspect of the present invention, the chargedparticles having a intensity distribution to make the inclination of thedistribution of the reflected electron intensity flat are irradiated inan inclined reflected electron intensity region in a latent imagepattern formed on the resist. Therefore, since an amount of the chargedparticles required for resolving the image is supplied in the regionwherein the exposure amount becomes insufficient because of thereduction of the distribution of the reflected electron intensity, thepattern of the latent image with high accuracy can be formed on theresist.

Further, in a case where the charged particles having differentdistribution of the electron beam intensity are irradiated, the exposuremask which has a plurality of electron beam transmission holes, eachhaving an aperture region in proportional to a transmission electronbeam amount, may be used. Therefore, the charged particles havingunequal distribution of the electron beam intensity can be irradiated onthe resist even by one electron beam shot.

Furthermore, before such exposure mask is formed, patterns to be exposedon the resist are first divided into a plurality of rectangular regions,then the reflected electron intensities are then measured at fourcorners of each rectangular region, then changes in reflected electronintensities are checked by comparing reflected electron intensitiessequentially, and then change rates are calculated if the change of thereflected electron intensities are found. In turn, electron beamtransmitting holes having their size in proportional to the change ratesare formed on the exposure mask. Like this, sizes and dimensions of theelectron beam transmitting holes can be determined relatively readily.

According to still another aspect of the present invention, using theexposure mask having a plurality of electron beam transmitting holesformed in a matrix fashion, non-pattern forming regions of the positivetype resist, for example, are exposed. Therefore, by reducing theexposure amount of the non-pattern forming regions, the proximity effectaffected on adjacent pattern forming regions can be suppressed, so thatthe patterns are formed on the resist with high accuracy.

In this case, the patterns can be formed with higher accuracy by settingthe different exposure amount respectively on the pattern forming regionand the non-pattern forming region.

In addition, in the exposure mask having the plurality of electron beamtransmitting holes formed in the non-pattern forming regions in a matrixfashion, an entire region including the pattern forming region and thenon-pattern forming region is divided into plural sections, and sizesand pitches of the electron beam transmitting holes are determined basedon ratios of exposure amount/area in these sections. Thus it can beeasily determined whether or not the pattern forming region isover-exposed. It is another object of the present invention to provide acharged particle beam exposure mask capable of preventing degradation inpattern precision due to proximity effect without lowering throughput ofexposure process, and to provide a method of forming a mask allowing anamount of charged particles due to proximity effect to be corrected.

In the charged particle beam exposure mask according to the presentinvention, one pattern is partitioned into a plurality of rectangles,and transmission holes of large size are formed in respective rectanglesof the plurality of rectangles wherein an amount of charged particlesdue to proximity effect is small, whereas transmission holes of smallsize are formed in respective rectangles wherein an amount of chargedparticles due to proximity effect is large. As a result, an irradiatingamount of charged particles can be controlled, so that bulge, reductionor elimination of transfer patterns due to proximity effect can beremoved. Latent images can therefore be formed on a resist with goodprecision.

Furthermore, if the charged particle exposure mask is employed, theexposure process can be accomplished at a time to thus improvethroughput. Since respective transmission holes are partitioned by meansof frames, strength of the mask is in no way lowered. The frames may beformed by the mask substrate.

Moreover, when such a charged particle beam exposure mask is fabricated,first an amount of charged particles due to proximity effect is inadvance calculated in respective regions, then sizes of transmissionholes are contracted in respective regions wherein an amount of chargedparticles due to proximity effect is large while sizes of transmissionholes are magnified in respective regions wherein an amount of chargedparticles due to proximity effect is small. As a result, with takinginfluence of proximity effect caused in neighboring patterns as well asthe concerned pattern itself into consideration, sizes of respectivetransfer holes can be easily determined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view showing a pattern of the first conventionalexposure mask;

FIG. 1B is a plan view showing another pattern of the first conventionalexposure mask;

FIG. 2A is a plan view showing a pattern of the second conventionalexposure mask;

FIG. 2B is a plan view showing another pattern of the secondconventional exposure mask;

FIG. 3A is a plan view showing a pattern of the third conventionalexposure mask;

FIG. 3B is a plan view showing another pattern of the third conventionalexposure mask;

FIG. 4 is a plan view showing an aperture pattern of the first exposuremask used in an experiment according to the present invention;

FIG. 5 is a microphotograph showing a plan view of the exposed resistpattern obtained when the exposure mask shown in FIG. 4 is employed;

FIG. 6 is a plan view showing an aperture pattern of the second exposuremask used in the experiment according to the present invention;

FIG. 7 is a microphotograph showing a plan view of the exposed resistpattern obtained when the exposure mask shown in FIG. 6 is employed;

FIG. 8 is a plan view showing an aperture pattern of the first exposuremask according to the first embodiment of the present invention;

FIG. 9 is a microphotograph showing a plan view of the exposed resistpattern obtained when the exposure mask shown in FIG. 8 is employed;

FIG. 10 is a plan view showing an aperture pattern of the secondexposure mask according to the first embodiment of the presentinvention;

FIG. 11 is a microphotograph showing a plan view of the exposed resistpattern obtained when the exposure mask shown in FIG. 10 is employed;

FIG. 12 is a plane view showing an aperture pattern of the thirdexposure mask according to the first embodiment of the presentinvention;

FIG. 13 is a microphotograph showing a plan view of the exposed resistpattern obtained when the exposure mask shown in FIG. 12 is employed;

FIG. 14 is a plan view showing an aperture pattern of the fourthexposure mask according to the first embodiment of the presentinvention;

FIG. 15 is a view schematically illustrating a configuration of anexample of an exposure apparatus in which the exposure masks of thepresent invention are employed;

FIG. 16 is a diagram showing a distribution of exposure amount toillustrate a proximity effect caused by electron beams in the presentinvention;

FIG. 17 is a view showing an exposure limit if the proximity effect istaken into consideration;

FIG. 18 is a plan view of an exposure mask used for determining variousconditions of forming patterns of the exposure masks according to thefirst embodiment of the present invention;

FIG. 19 is a plan view showing a relation between a width of a crossbeamand a bulge of the pattern of the exposure masks according to the firstembodiment of the present invention;

FIG. 20 is a plan view showing the first example of exposure masks usedfor an electron beam exposure method according to the second embodimentof the present invention and an auxiliary exposure region thereof;

FIG. 21 is a view showing a distribution of electron intensity on theresist when an auxiliary exposure is not effected;

FIG. 22 is a view showing a distribution of electron intensity on theresist when an auxiliary exposure is effected;

FIG. 23 is a plan view showing a situation where the second exposuremask used for the electron beam exposure method according to the secondembodiment of the present invention is divided into a plurality ofrectangular regions by virtual lines;

FIG. 24 is a view showing a distribution of electron intensity on theresist when exposed by employing the exposure mask in FIG. 23;

FIG. 25 is a view showing a distribution of electron intensity on theresist when the auxiliary exposure shown in FIG. 24 is effected;

FIG. 26 is a view showing a distribution of electron intensity toillustrate a change in reflected electron intensity on the resist whenexposed by employing the exposure mask in FIG. 23;

FIG. 27 is a view showing a distribution of electron intensity obtainedby the auxiliary exposure;

FIG. 28 is a plan view showing an auxiliary exposure mask used for theelectron beam exposure method according to the second embodiment of thepresent invention;

FIG. 29 is a view showing a distribution of electron intensity toillustrate a shading caused around the electron beam irradiated region;

FIG. 30 is a view showing a distribution of electron intensity toillustrate a situation where no shading is caused around the electronbeam irradiated region;

FIG. 31 is a plan view showing part of a pattern to be formed on theresist;

FIG. 32 is a plan view showing part of an exposure mask used for theelectron beam exposure method according to the third embodiment of thepresent invention;

FIG. 33 is a plan view showing an example of a size of island-likepatterns shown in FIG. 32;

FIG. 34 is a plan view showing an example of a size of holes formed in amatrix fashion and disposed in pattern non-forming regions;

FIG. 35 is an enlarged plan view showing a hole in FIG. 34 and thecrossbeam formed around the hole;

FIG. 36 is a characteristic diagram showing a relation between anaperture region and an amount of current passing through the exposuremask;

FIG. 37 is a plan view showing a distribution of charged particles inthe exposure mask shown in FIG. 32;

FIG. 38 is a microphotograph showing a resist pattern formed via theexposure process using the exposure mask used for the electron beamexposure method according to the third embodiment of the presentinvention;

FIGS. 39A and 39B are microphotographs showing respectively enlargedparts of the pattern of the photograph in FIG. 22;

FIG. 40 is a microphotograph showing the resist pattern formed via theexposure process without the exposure mask used for the electron beamexposure method according to the third embodiment of the presentinvention;

FIGS. 41A and 41B are microphotographs showing respectively enlargedparts of the pattern of the photograph in FIG. 39.

Preferred embodiments of the present invention will now be describedhereinafter with reference to the accompanying drawings;

FIG. 42 is a flow chart of the manufacturing steps of the auxiliaryexposure mask of the second embodiment of the present invention;

FIG. 43 is a fragmental sectional view illustrative of the proximityeffect in the exposure object;

FIG. 44 is a view showing exposure amount and resultant transferpatterns in connection with the conventional mask arrangements;

FIGS. 45A and 45B are plan views showing respectively a method offorming a mask according to a first example of a fourth embodiment ofthe present invention;

FIG. 46 is a view showing exposure amount and resultant transferpatterns in connection with mask arrangements in FIG. 45B according tothe first example of the fourth embodiment of the present invention;

FIG. 47 is a fragmental plan view showing transfer patterns according toa second example of the fourth embodiment of the present invention;

FIG. 48 is a plan view showing the transfer patterns of the block shownin FIG. 47 in an enlarged fashion;

FIG. 49 is a plan view showing the mask pattern used to form thetransfer patterns shown in FIG. 48;

FIGS. 50 and 51 are plan views showing respectively a method of formingrectangular patterns of a mask according to a third example of thefourth embodiment of the present invention; and

FIGS. 52 and 53 are plan views showing respectively the method offorming the rectangular patterns of the mask according to the thirdexample of the fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment)

The first embodiment of the present invention will be described belowwith reference to FIGS. 4 to 19.

(1) Explanation of a pattern of an exposure mask according to the firstembodiment:

FIGS. 4, 6, 8, 10, 12 and 14 are plan views showing patterns of electronbeam exposure masks according to the first embodiment of the presentinvention. FIGS. 5, 7, 9, 11 and 13 are microphotographs showingpatterns of EB resists obtained by employing these exposure masks.

In order to obtain an optimal pattern of the exposure mask, variousexperiments have been tried by the inventors of the present invention.

At first, in a method which has been proposed as a block exposure (orcell projection), it is confirmed whether a "distance less than aresolution limit" could be obtained on the exposure mask (which is alsoreferred to as the transmission mask).

In the exposure mask described, a wide concave portion is formed on asilicon substrate to form a thin film region, and the thin film regionis used as a membrane. With taking account of mechanical strength,electron stopping capability, and the like, a thickness of the membraneis determined as a lower limit value, i.e., 20 μm.

Furthermore, within a one shot range of the electron beam, one or morenon-exposed patterns, each having an island-like or peninsula-like planshape, and their peripheral ring or U-shaped opening portions fortransmitting charged particles (electron beams) are formed in theexposure mask. In addition, bridging portions (i.e., crossbeams), eachhas a width which is not exposed substantially on a wafer, are disposedin beam transmitting regions so as to hold the island-like orpeninsula-like non-exposed patterns.

(a) First exposure mask and exposure results obtained by the firstexposure mask:

As shown in FIG. 4, a pattern formed on the membrane is employed as thefirst exposure mask. The exposure mask has patterns which are used toform rectangular patterns successively on a positive type EB resist.

In FIG. 4, a plurality of opening regions 3, 4 are formed on divisionlines which divide the exposure mask into a plurality of rectangularelectron beam shielding patterns 1 as island-like patterns, each havinga size 40 μm×100 μm. The first two rectangular opening regions 3 eachhaving a width (W1) of 15 μm are formed on two longer sides of theelectron beam shielding patterns 1. The two opening regions 3 areseparated by a bridging portion (bridging non-opening portion) 2 havinga length (W0) of 8 μm. The second opening region 4 having a width (W) of15 μm is formed on two shorter sides of the electron beam shieldingpatterns 1. Virtual lines, which divide the exposure mask into theelectron beam shielding patterns 1 in vertical and horizontaldirections, can be considered to pass on center lines of the firstopening regions 3 and the second opening regions 4.

In addition, the third rectangular opening regions 5 are formed inintersecting regions wherein alignments of the first opening regions 3intersect the alignments of the second opening regions 4. Each of thethird opening regions 5 serves to separate four corners of adjacentelectron beam shielding patterns 1. Each of the third opening regions 5is separated by bridging portions 6 from the first opening regions 3 andthe second opening regions 4.

When the positive type EB resist is exposed by the electron beams byemploying the exposure mask formed above, a positive type EB resistpattern shown in the microphotograph of FIG. 5 can be derived. As aresult, it has been found that the patterns corresponding to thebridging portions 2 each having a length of 8 μm are resolved on the EBresist even by the electron beams.

Since a pattern magnification of the exposure mask is set to 100 times,the lengths W0 of the bridging portions 2 and 6, each respectivelyseparating the first opening regions 3 from each other and separatingthe third opening regions 5 from the first and second opening regions 3and 4, become 8 μm on the exposure mask and become 0.08 μm on the EBresist.

Now, the length W0 of the bridging portion (frame) is defined as alength of the bridging portion in the direction along the edge line ofthe electron beam shielding patterns 1 (i.e., the alignment direction ofthe opening portion). The width W1 of the opening region is also definedas a length of the opening region in the direction perpendicular to theedge line of the electron beam shielding patterns 1 (i.e., the alignmentdirection of the opening region). The above definitions can also besuitable for following explanations.

(b) Second exposure mask and exposure results obtained by the secondexposure mask:

FIG. 6 shows part of a pattern of the second exposure mask. The secondexposure mask has the same pattern configuration as that of the firstexposure mask, except that lengths Wo of bridging portions 2 and 6, eachrespectively separating the first opening regions 3 from each other andseparating the third opening regions 5 from the first and second openingregions 3 and 4, are formed as 6 μm.

While supplying an exposure amount slightly larger than the optimalexposure amount to the positive-type EB resist, a positive type EBresist pattern shown in FIG. 7 can be derived when the resist isover-exposed by the electron beams with employing the second exposuremask.

In this second exposure mask, it has been confirmed that, unlike thefirst exposure mask, the patterns corresponding to the bridging portions2 and 6 are not resolved, but their traces still remain on the EB resistpatterns. In consequence, it has been found that, if the lengths Wo ofthe bridging portions 2 and 6 are narrowed to about 6 μm, they are notresolved on the positive type EB resist.

(c) Third exposure mask and exposure results obtained by the thirdexposure mask:

FIG. 8 shows part of a pattern of the third exposure mask. The thirdexposure mask has the same pattern configuration as that of the firstexposure mask, except that lengths Wo of bridging portions 2 and 6, eachrespectively separating the first opening regions 3 from each other andseparating the third opening regions 5 from the first and second openingregions 3 and 4, are formed as 2 μm.

While providing an exposure amount slightly larger than the optimalexposure amount to the positive type EB resist, a positive type EBresist pattern shown in FIG. 9 can be derived when the resist isover-exposed by the electron beams with employing the third exposuremask.

According to the third exposure mask, not only the patternscorresponding to the bridging portions 2 have not been resolved, butalso their traces have not been found. Thus linear stripe-likeclearances are formed around the pattern corresponding to the electronbeam shielding pattern 1, due to the proximity effect caused by theelectron beams, described later.

(d) Fourth exposure mask and exposure results obtained by the fourthexposure mask:

As shown in FIG. 10, a pattern formed on the membrane is used as thefourth exposure mask.

As in the first to third exposure masks, the fourth exposure mask haspatterns which are used to form rectangular patterns on thepositive-type EB resist.

In FIG. 10, a plurality of opening regions 12, 13 are formed on virtuallines which divide the exposure mask into a plurality of rectangularelectron beam shielding patterns 11 as island pattern regions, eachhaving a size 40 μm×100 μm. A plurality of the first rectangular openingregions 12, which are separated via bridging portions (crossbeams) 14from each other, are formed on longer sides of the electron beamshielding patterns 11. A plurality of the second opening regions 13,which are separated via a bridging portion 15 from each other, areformed on shorter sides of the electron beam shielding patterns 11. Thesides of the first opening region 12 and the second opening region 13 inthe direction along the edge lines of the electron beam shieldingpatterns 11 are 6 μm in length. Also the sides of the first openingregion 12 and the second opening region 13 in the directionperpendicular to the edge lines of the electron beam shielding patterns11 are 11 μm in length. All the distances between the first openingregions 12, the distances between the second opening regions 13, thatis, the lengths Wo of the bridging portions 14, 15 are set to 2 μm.

Virtual lines dividing the exposure mask into the electron beamshielding patterns 11 can be considered to pass on center lines of thefirst opening regions 12 and the second opening regions 13.

In this case, the pattern magnification of the exposure mask is set to100 times. Widths of the first opening regions 12 and the second openingregions 13 which are formed to surround the electron beam shieldingpatterns 11 are 11 μm on the exposure mask and are about 0.11 μm on theEB resist.

FIG. 11 is a microphotograph showing a pattern of the EB resist whereinit is over-exposed by 90 μC/cm² which corresponds to about 30%over-exposure.

In this case, the lengths of the bridging portions 14, 15 become about0.02 μm on the EB resist formed on the wafer. However, even if theover-exposure is effected by about 20%, the bridging portions 14, 15have not been resolved. As a result, the resist could be removed instraight stripe fashion from the edge lines of the rectangular patternscorresponding to the electron beam shielding patterns 11.

In the fourth mask, in contrast to the first to the third masks, a largenumber of the first opening regions 12 and the second opening regions13, both having short sides, are formed. Therefore, it has beenconfirmed experimentally that a linearity of edge lines of therectangular patterns corresponding to the electron beam shieldingpatterns 11 on the EB resist can be improved.

(e) Fifth exposure mask and exposure results obtained by the fifthexposure mask:

As shown in FIG. 12, a pattern formed on the membrane is used as thefifth exposure mask. Employing the fifth exposure mask, rectangularpatterns are formed on the positive type EB resist.

In FIG. 12, a plurality of opening regions 22, 23 are formed on virtuallines which divide the exposure mask into a plurality of rectangularelectron beam shielding patterns 21 as island-like patterns, each havinga size 40 μm×100 μm. A plurality of the first rectangular openingregions 22, which are separated via bridging portions (bridge-likenon-opened portions) 24 from each other, are formed on longer sides ofthe electron beam shielding patterns 21. A plurality of the secondopening regions 23, which are separated via bridging portions 25 fromeach other, are formed on shorter sides of the electron beam shieldingpatterns 21.

The sides of the first opening region 22 and the second opening region23 in the direction along the edge lines of the electron beam shieldingpatterns 21 are 6 μm in length. Also the sides of the first openingregion 22 and the second opening region 23 in the directionperpendicular to the edge lines of the electron beam shielding patterns21 are 13 μm in length. All the distances between the first openingregions 22, the distances between the second opening regions 23, thatis, the lengths Wo of the bridging portions 24, 25 are set to 4 μm.

Virtual lines, which divide the exposure mask into the electron beamshielding patterns 21, can be considered to pass on center lines of thefirst opening regions 22 and the second opening regions 23.

In addition, the third rectangular (square) opening regions 26 aredisposed in intersecting regions wherein alignments of the first openingregions 22 intersect the alignments of the second opening regions 23.Each side of the third opening regions 26 is set to 9 μm in length whichis smaller than widths of the first opening regions 22 and the secondopening regions 23.

At that time, even if the over-exposure is made by about 15%, thebridging portions 24, 25 have not been resolved. FIG. 13 is amicrophotograph showing a pattern of the EB resist wherein it ispatterned by over-exposing of 80 μC/cm² which corresponds to about 20%over-exposure, employing the fifth exposure mask. The portionscorresponding to the bridging portions 24, 25 have not been resolved onthe resist, so that good rectangular patterns having shapescorresponding to the electron beam shielding patterns 21 could bederived.

In the meanwhile, since bridging portions 27 are formed around the thirdopening regions 26 by disposing the third opening regions 26 in theintersecting regions, mechanical strength of the intersecting regions inthe exposure mask can be increased.

If cross-shaped opening patterns are formed instead of the bridgingportions 27 in the intersecting regions, corners of the rectangularpatterns are extremely rounded off due to the proximity effect caused bythe electron beams. However, in the fifth exposure mask, since thebridging portion 27 serves to reduce an exposure amount in theintersecting regions, it can be suppressed that the corners of thepatterns are rounded off. This is apparent from the exposure resultshown in FIG. 13, for example.

This fact is true of the first to fourth exposure masks. However, if fewopening regions are disposed around the electron beam shielding patterns21, i.e., if the opening regions are set to be long, central portions ofthe opening regions are bulged, as shown in FIGS. 5 and 7. In addition,the more the lengths of the opening regions are shortened, i.e., themore the number of the bridging portions are increased, the more thelinearity of the pattern edges on the EB resist can be improved, asshown in FIGS. 11 and 13.

Various conclusions are derived from the above experimental results asstated in the following.

Since the bridging portions of the exposure mask must not be resolved bythe EB exposure on the positive type EB resist which is formed on thewafer, the widths of the bridging portions cannot be set to largewithout any restriction. In this case, the more the bridging portionsare arranged narrowly, the more the thermal and mechanical strengths canbe increased. On the contrary, if the bridging portions are formed toomuch in number, the gross area of the shielding patterns is increased,so that the exposure amount becomes insufficient on the resist.

According to the experiments performed by the present inventors, it hasbeen found that the ratio (W/Wo) of lengths W of the opening regionsagainst the lengths Wo of the bridging portions must be required 1.5times or more at the lowest. For instance, if the length Wo of thebridging portion is set to 4 μm, the length W of the beam transmittingregion (the length of the opening region) is required 6 μm or more. Inother word, the pitch of the bridging portions must be set to be 2.5times or more as large as the length W.

Accordingly, the electron beam exposure mask where the length W of thebeam transmitting region is set to be longer 1.5 times or more than thelength Wo of the bridging portion within an exposed range, or theelectron beam exposure mask where the pitch of the bridging portion isset to be longer 2.5 times or more than the width of the bridgingportion is needed.

Although the case where the island-like patterns are formed on theexposure mask has been explained as above, an example of the patterns ofthe exposure mask which is employed to form the peninsula-like patternson the EB resist is shown in FIG. 14. In this example, good exposure asdescribed above can also be attained if the a plurality of openingregions 31 are disposed along the U-shaped pattern. Note that, in thiscase, since the opening region 31a arranged on the corners of theU-shaped pattern has small swell due to the proximity effect, it may beformed in the same width W3 as that of other opening regions 31.Patterns in the exposure mask described above can be formed on theresist by one electron beam shot.

(2) Relationship between a pattern width of the exposure mask accordingto the first embodiment of the present invention and the proximityeffect:

The exposure masks described above are employed when the resist isexposed by means of the having a structure shown in FIG. 15. In theelectron beam exposure apparatus, the electron beam (charged particlesbeam) is emitted from a cathode electrode K, and is passed through agrid G, an anode A, a beam shaping slit BS1, a focusing lens L1, a slitdeflector for deflecting a beam location, a second lens L4, and theexposure mask, and is then patterned. Then, the electron beam is passedvia lenses L2 and L3, an aperture AP etc. and is then irradiated on theEB resist R formed on the surface of a wafer W to thus form a latentimage thereon. In FIG. 15, a reference TD denotes a deflector whichdeflects the electron beam to select patterns on the exposure mask.

After completing the exposure as described above, the patterns areformed as visible images by developing the EB resist as described above.

In the meanwhile, in order to eliminate the pattern of the bridgingportions of the exposure mask, it is requested that the bridgingportions are not resolved because of the proximity effect caused by theelectron beams which transmit near the opening regions (beamtransmitting regions) around the bridging portions. In other words, thebridging portions of the exposure mask does not appear as the patternsbecause of not the diffraction wave effect but the proximity effect ofthe exposure mask at the time of resist development.

The proximity effect is defined as that exposure regions on the resistare spread by scattering of the charged particles when they areirradiated on the resist. The scattering is classified into a forwardscattering and a backward scattering. The former is denoted as that thecharged particles irradiated onto the resist scatters within the resist.The latter is denoted as that charged particles, which enter into thewafer via the resist, scatters by rebounding on the wafer or springingback.

Depending upon the accelerating voltage of the electron beam etc., arange affected by the proximity effect is determined. A range of about0.01 to 0.1 μm is influenced by the forward scattering, and a range ofabout 3 to 5 μm is influenced by the backward scattering. Since theforward and backward scatterings have Gaussian distribution commonly,the factor which the resist receives actually, i.e., the factor ofdecreasing a resolution ability of the electron beam is given as a sumof the forward scattering and the backward scattering, as shown in FIG.16. However, as shown in FIG. 17, depending that an amount of exposureis excess of a boundary value (also called as a thresholds value), it isdetermined whether or not the pattern is resolved on the resist.Therefore, edge portions of the pattern are determined according to thelevel of the boundary value. If the amount of exposure exceeds theboundary value, the pattern is resolved.

In general, if the opening patterns for transmitting the chargedparticles are disposed closely more and more in the exposure mask,overlappings of the charged particles (exposure amount) caused by theirproximity effects tends to form their united opening patterns on theresist. For example, as shown in FIG. 18, if the resist is exposed byusing the exposure mask where opening regions 42, 43, both spaced apartby a distance of 4 μm, are disposed around a rectangular electron beamshielding pattern 44 via bridging portions 41 in a double line, a unitedpattern of two opening regions 42, 43 are formed on the resist due tothe proximity effect, although not shown. However, if two openingregions 42, 43 are formed in a double line as above, the proximityeffect caused by the right side opening region 42 affects the left sideregion of the left side opening region 43, for example, to result inspread pattern widths on the resist. In consequence, such double linearrangement is not preferable for the exposure mask since thedimensional accuracy is degraded.

Therefore, if, as described, the proximity effect is taken into account,such a condition is required that, when the island-like orpeninsula-like patterns are surrounded by a plurality of openingregions, the opening regions must not be arranged in parallel.

In the meanwhile, as shown in FIG. 19, a length Wo of the bridgingportion 51 of the exposure mask is so determined that the bridgingportion 51 is not resolved on the resist because of the proximityeffect. Since the proximity effect affects the region of the openingregion 52 where the bridging portion 51 is not formed, a bulge (shiftcomponent) W21 of the width of the pattern of the opening region 52 iscaused on the EB resist. Besides, the exposure amount tends to beincreased in the bridging portion 51 of the exposure mask, because of asynergetic effect of the proximity effect caused by two opening regions52 disposed in both sides of the bridging portion 51. Accordingly, ifthe length of the bridging portion 51 is set slightly longer than twotimes of the bulge W21 of the opening region 52, the bridging portion 51is not resolved on the resist, and therefore it does not appear as thepattern on the resist.

For instance, if the bulge W21 of the opening region 52 caused by theproximity effect on the EB resist and the width of the pattern to beexposed is set to 0.15 μm, the width W1 of the opening region 52 may beset 0.11 μm by adding the bulges of both sides of the opening region 52.In this case, when the length Wo of the bridging portion 51 is set to0.04 μm, the bridging portion 51 does not appear as the pattern on theresist.

The resist has been exposed indeed by the exposure mask fabricated basedon the analysis above. When the width of the pattern, which correspondsto the opening region 52 located in the region irradiated by the chargedparticles, becomes 0.15 μm, the pattern of the bridging portion 51 isnot eliminated and still remains on the resist. Furthermore, when theexposure amount is increased so as not to resolve the bridging portion51, the pattern width W1 of the opening region 52 becomes 0.20 μm tothus cause an actual bulge larger by 0.05 μm than an estimated bulge.

Thereby, it has been found that it is hard to determine the patternwidth of the opening region only by the analysis. Therefore, the bulgeW21 of the pattern is detected in advance to collect data when thepattern of the bridging portion 51 disappears. The width W1 of theopening region (beam transmitting hole) 52 must be set to be smaller(offset) than a width based on the data. For instance, the bulge W21 ofthe opening region 52 is set larger than the length Wo of the bridgingportion 51 as an offset amount.

(3) Shape of four corners of the electron beam shielding region in theexposure mask of the embodiment of the present invention:

As has been described advance, if a plurality of opening regions aredisposed in two directions (e.g., vertical and horizontal directions),the opening region, if located in the intersecting region of twodirections and formed as a cross shape, causes the proximity effect morethan other portions. Thus an over-exposure is produced, and the cornerportion of the rectangular region which is surrounded by a plurality ofopening regions is formed as a round portion. On the other hand, if notransmitting hole is provided in the intersecting region (or if thetransmitting hole is too small), acute angle corner portions shown inFIG. 5 are formed in the resist pattern of adjacent rectangular region.Thus it becomes difficult to form desired patterns. As a result, a holehaving a suitable size for transmitting the electron beam must be formedindispensably in the central portion of the intersecting region.

In case three or more adjacent island-like patterns are disposed in theexposure mask, beam transmitting holes shown in FIG. 10 or FIG. 12 mustbe formed in the center of the intersecting region wherenon-transmitting holes are adjoined each other in the exposure mask.

(4) Area of the opening regions by one shot employing the exposure maskof the embodiment of the present invention:

In case the exposure is performed in fact by using the exposure mask,the exposure amount is slightly increased on the ordinary exposureregion. Although the bridging portion is eliminated by the proximityeffect on the EB resist, influences of the proximity effects vary unlessareas of the opening regions except for the above intersecting regionsare fixed constantly. Consequently, particular portions of the resistare over-exposed, or dimensional accuracy cannot be assured in thewhole, so that intended patterns are not derived.

Thus, in the patterns of the exposure mask, each of opening regions(beam passing regions) corresponding to exposed regions on the resist,i.e., respective opening region formed between the bridging portions,must be formed to have substantially the same opening area in a regionirradiated by the electron beam at a time (one electron beam shotrange).

In addition, in the patterns of the exposure mask, the width of each ofthe bridging portions corresponding to the non-exposed portions on theresist (beam shaded portion) must be formed to have substantially thesame length Wo in one electron beam shot range.

(Second Embodiment)

The second embodiment of the present invention will be described withreference to FIGS. 20 to 31 hereinbelow. (1) In the exposure mask (alsoreferred to as the block mask) of the first embodiment described above,the island-like or peninsula-like patterns are prevented from being comeout from the exposure mask, by dividing the opening region surroundingthe patterns by a plurality of crossbeams (electron beam shieldingregion). A technique for dividing the opening region by the crossbeamscan be employed inside of one rectangular opening pattern 61 formed inthe exposure mask 60, as shown in FIG. 20, as well as the above openingregions around the island-like or peninsula-like patterns. In otherwords, when an amount of the electron beam passing through the openingregions 61 formed at a predetermined location becomes too much, itbecomes possible to reduce the exposure amount (current amount of theelectron beam) passing therein by dividing the inside of the openingpatterns 61 into plural regions. In this case, the width of thecrossbeam 62 must be formed so small not to be resolved on the resist,as in the first embodiment.

While, the charged particles (or electron intensity) passing through thesame opening pattern 61 and irradiated on the resist (not shown) havesometimes different distributions on the resist because of the proximityeffect. Namely, as shown in FIG. 21, an amount of the charged particlesis reduced in edges and their periphery of the opening regions 61 incontrast to other regions. This is because few charged particles enterfrom the outside of the opening regions 61 by proximity effect.

As shown in FIG. 21, in case the electron intensity passing through nearthe edge of the opening regions 61 does not become larger than theresolving boundary value shown in FIG. 21, narrow patterns are formed onthe resist.

Therefore, in the second embodiment, the electron intensity on theresist is corrected by irradiating the electron beam partially in theresist region (region encircled by the broken line in FIG. 20) whichcorresponds to the edges and their periphery of the opening regions 61.Such partial irradiation of the electron beam is denoted as auxiliaryexposure or inclined auxiliary exposure hereinafter. The auxiliaryexposure is effected before or after the entire exposure of the openingpattern.

According to the auxiliary exposure, as shown in FIG. 22, the chargedparticles can be increased partially and adequately in the region of theedges of the opening regions 61 and their periphery. Thereby, theopening pattern 61 can be transferred onto the resist as a latent image(not shown) with high accuracy.

Unless the inside of the opening regions 61 of the exposure mask isdivided by the crossbeams 62, the inclined auxiliary exposure is needed.This is because the distribution of the charged particles becomes smallin the region of the edges of the opening regions 61 and their peripheryeven when the crossbeams 62 are not formed in the opening regions 61.

Thus, in case the exposure mask having opening patterns withoutcrossbeams therein is employed, the auxiliary exposure will be explainedwith reference to examples in detail hereinbelow.

(2) FIG. 23 shows a rectangular opening pattern 61A formed in theexposure mask which is formed of a silicon plate. The opening patternhas a size of Kx×Ly.

Such opening pattern 61A is exposed by one electron beam shot. But, inorder to form the patterns with high accuracy, there exists some caseswhere the auxiliary exposure as above is required. A method of formingthe exposure mask used for the auxiliary exposure will be describedhereinafter as shown in FIGS. 23 to 28 and FIG. 42.

First, as shown by the broken line in FIG. 23, the opening pattern 61Aand its peripheral region is divided into a plurality of regions X1 toX43 by lattice-like virtual lines. Note that the virtual lines may bedrawn either to overlap the edge of the opening pattern 61A or not tooverlap the same. One divided region is formed as a rectangular block,and black round marks a to t indicate four vertexes of the rectangularblocks. At least three vertexes of the rectangular blocks are overlappedvertexes of adjacent rectangular blocks. It is of course that therectangular block is smaller than Kx×Ly.

After the edges of the opening regions 61A and their periphery aredivided into rectangular blocks, reflected electron intensities arecalculated on all vertexes a to t. Reflected electron intensities causedby the proximity effect are shown in FIG. 24, for example. In FIGS. 24and 25, the reflected electron intensities in three regions X22, X23 andX42 divided as shown in FIG. 23 and the electron intensity of theelectron beam irradiated in the center among their regions areillustrated.

According to the result of such calculation as shown in FIG. 24, it canbe seen that the reflected electron intensity become smaller toward theoutside of near the edge portion of the opening pattern 61A.

After the reflected electron intensities on all vertexes are calculated,changes of reduction of the reflected electron intensities arecalculated in every rectangular block.

At first, it is determined whether, in divided regions (rectangularblock regions) X1 to X43, the reflected electron intensities areclassified into 2 types or 3 types or more.

The 2 types classification denotes that the reflected electronintensities on four vertexes of one rectangular region are divided into2 electron intensity types. In this case, only a case where two linesconnecting vertexes having the same reflected electron intensity (forexample, L1 and L2 in the region X42) becomes parallel is considered. Inother cases, patterns for the auxiliary exposure are not formed. A slantA of reflected electron intensity is calculated between two vertexeshaving different reflected electron intensities in the directionperpendicular to the above two lines. A rate of change of the reflectedelectron intensity (slant A) is a value that is obtained by dividing adifference in the reflected electron intensities by a distance betweenthe vertexes.

On the other hand, the 3 types classification denotes that the reflectedelectron intensities on four vertexes of one rectangular region aredivided into 3 or 4 electron intensity types. In this case, two vertexeseach having maximum and minimum reflected electron intensities are firstselected, and then a rate of change of the reflected electron intensity(slant A) is calculated by dividing a difference in the reflectedelectron intensities by a distance between the vertexes. For example, inthe rectangular region X43 in FIG. 23, the reflected electronintensities are divided into three types or more. If the maximum valueand the minimum value of the reflected electron intensities are on adiagonal line of the region X43, the reflected electron intensities hasthe distribution roughly shown in FIG. 26.

Meanwhile, in both cases of the 2 type classification and the 3 typeclassification of the reflected electron intensities, the rates ofchanges of the reflected electron intensities may be calculated by thefollowing equation (1). Now, assume that a larger reflected electronintensities is set to Rmax and also a smaller reflected electronintensity is set to Rmin on two vertexes, and a distance between twovertexes is set to B.

    A =(Rmax-Rmin)/B                                           . . . (1)

The mask for the inclined auxiliary exposure is made of the siliconplate. As described above, in the regions where the reflected electronintensities are classified into two kinds (for example, X42) and wherethe reflected electron intensities are classified into three kinds ormore (for example, X43) on the silicon plate, a plurality of holes areformed described hereinbelow.

As shown in FIG. 28, the mask for the auxiliary exposure has a pluralityof rectangular holes in a region corresponding to the rectangular regionin FIG. 23. Theses holes have different open regions to compensate theinclination of the reflected electron intensities as shown in FIG. 26 tosubstantial zero. For instance, in FIGS. 23, 26, the holes having thelargest region are formed in the portion corresponding to the vertex tof Rmin required for the largest inclined auxiliary exposure, and theholes having the smallest region are formed in the portion correspondingto the vertex o of Rmax. A plurality of holes, each having differentarea in proportional to the inclination A, are disposed between thesetwo holes. Thereby, in the auxiliary exposure mask, the electrons aretransmitted at the largest amount near the vertex t whereas theelectrons are transmitted at the smallest amount near the vertex o.

Assume that, in one rectangular region of the auxiliary exposure mask,an area of the smallest hole (65a) is set to 1, an area of the largesthole (65e) is set to S, and an exposure amount (current value of theelectron beam) is set to R, an exposure amount for suppressingdegradations of the reflected electron intensities can be derived fromthe following equation (2).

    SR+Rmin=R+Rmax                                             . . . (2)

Then an area S of the largest hole (65e) can be derived from thefollowing equation (3).

    S= (Rmax-Rmin)/ R!+1                                       . . . (3)

In the region between the smallest hole (65a) and the largest hole(65e), an area Sx of the hole spaced apart by a distance D from thesmallest hole (65a) can be derived from the following equation (4).

    Sx=(AD/R)+1                                                . . . (4)

These calculations are effected in the regions where the reflectedelectron intensities are classified into two kinds and where thereflected electron intensities are classified into three kinds or moreindividually.

From the relationship described above, the auxiliary exposure mask 63used for the exposure mask for forming the opening pattern 61A in FIG.23 is formed as shown in FIG. 28. More particularly, a plurality ofrectangular holes 64a to 64e shown in the lower right in FIG. 28 areformed in the region Y43 corresponding to the region X43 in FIG. 23. Aplurality of rectangular holes 65a to 65e each having a different sizeand shown in the lower left in FIG. 28 are formed in the region Y42corresponding to the region X42 in FIG. 23. In addition, a plurality ofholes are formed in other regions Y11 to Y13, Y21, Y23, Y31, Y33 andY41. However, since four vertexes have the same reflected electronintensity in the central regions X22 and X32 of the opening pattern 61A,the auxiliary exposure is not required in the two central rectangularregions Y22, Y32 shown in FIG. 28. Therefore, holes used for theauxiliary exposure are not formed. Since the auxiliary exposure has asmall amount of charged particles, the pattern are not resolved just bythe auxiliary exposure.

When employing the auxiliary exposure mask 63, the auxiliary exposure asshown in FIG. 27 is effected on the resist having the distribution ofthe reflected electron intensities in FIG. 26. As a result, thedistribution of the reflected electron intensities on the resist iscorrected from the state of FIG. 24 to the state of FIG. 25, so that aninclination of the reflected electron intensities in the regioncorresponding to the edge portion of the opening pattern 61A and itsperipheral portion can be made flat substantially. Thereby, thereflected electron intensities in the edge portion of the openingpattern 61A is increased in excess of the resolution boundary value tothus obtain a proper exposure amount. After effecting the auxiliaryexposure, a pattern accuracy can be improved on the resist. Theauxiliary exposure can be effected before or after the opening pattern61A is exposed. The auxiliary exposure can also be effected by theelectron beam exposure apparatus having a configuration shown in FIG.15.

When the auxiliary exposure being made, the auxiliary exposure iseffected by the exposure amount R in the region having the largestreflected electron intensities.

Therefore, if the opening pattern 61A shown in FIG. 23 is exposed, theexposure amount must be reduced by the exposure amount R.

Since the auxiliary exposure as described above is effected partially,an irradiating time of the electron beam required therefor is small sothat the auxiliary exposure would not cause so serious degradation ofthe throughput of the exposure. In addition, since the auxiliaryexposure mask 63 has holes partially, the mechanical strength of theexposure mask is not lowered and is thus solid. Furthermore, since theauxiliary exposure mask has a plurality of holes each having a differentsize, the distribution of the exposure amount can be varied by oneelectron beam shot to therefore improve the pattern accuracy.

If the opening pattern having a not-rectangular shape is transferredonto the resist, the auxiliary exposure may be performed in the edge ofthe opening pattern and its peripheral region by employing the electronbeam having an inclined distribution.

In turn, an example of the method of calculating the reflected electronintensity will be explained hereinbelow.

At first, on the basis of the ratios of the exposure area calculated bydividing the opening pattern 61A into a plurality of rectangularregions, distances γ between measuring points a to t and the centerpoint of the rectangular regions are measured. And, the reflectedelectron intensity η in either one of the measuring points a to t can bederived from the following equation. ##EQU1## (Where, α, β, δ, κ, areconstants determined according to conditions such as kinds of theresist, thickness of the film etc.)

(Third Embodiment)

The third embodiment of the present invention will be described withreference to FIGS. 29 to 40B hereinbelow.

When the resist patterns are formed on the positive-type resist, theresist is smeared away with charged particles by irradiating theelectron beam in the regions where the resist must be removed. However,in case such regions are smeared away by the electron beam having alarge diameter, charged particles are sometimes spread into the patternforming region because of a coulomb interaction. Therefore, this spreadmust be prevented.

Thus, it can be considered that crossbeams are formed around the regionssmeared away by the electron beam to divide the regions into a pluralityof sections, as in the first embodiment. But, if the regions divided bythe crossbeams are wide, the coulomb interaction exerts a harmfulinfluence on the circumference, as shown in the distribution of theelectron intensity in FIG. 29. On the other hand, in case, under thesame current density of the electron beam, the exposure mask is dividedinto a plurality of narrow regions by the crossbeams, influence causedby the cross interaction effected from the charged particles irradiatedregions on the circumference can be scarcely found, as shown in FIG. 30.

FIG. 31 shows an island-like pattern 71 of the resist formed in a matrixfashion. The island-like patterns 71 are divided by grooves 72. A wideregion where no pattern is formed (pattern non-forming region) 73 isformed around the periphery of the collection region of the island-likepatterns 71. In this embodiment, in order to form the island-likepatterns 71 shown in FIG. 31, the exposure mask having opening patternsshown in FIG. 32 is used. The exposure mask shown in FIG. 32 is used forpatterning the positive-type resist.

In FIG. 32, a reference 74 denotes an island-like pattern surroundedopening regions 76 which are divided by the crossbeams (charged particleshielding regions) 75 as described in the first embodiment. A pluralityof island-like patterns 74 are formed in a matrix fashion. The pluralityof island-like patterns 74 are coupled to each other by crossbeams 75. Aplurality of small rectangular holes 77 are disposed around thecollection region of the island-like patterns 74 in a matrix fashion.Each of crossbeams 78 has a width which is not resolved on the resist.

Size of the matrix-like holes 77 are determined as follows.

First, a region where a plurality of island-like patterns 74 are formed(referred to as a pattern forming region hereinafter) is divided into aplurality of rectangular regions each of which corresponds to oneelectron beam shot region, and then a ratio of exposure area in everyrectangular region is calculated. Here the ratio of exposure area isdefined as a value which is derived by dividing a gross area used fortransmitting the electron beam within the rectangular region by therectangular region (i.e., region irradiated by the electron beam/area ofthe rectangular region).

In turn, sizes and pitches of matrix-like holes 77 are determined suchthat the ratio of exposure area of the pattern non-forming region 73becomes an equal value or a close value to the ratio of exposure areanear the outside of the pattern forming region. In this case, if thecharged particles passing through the plurality of holes 77 areirradiated onto the positive type resist during the exposure process,such close value is defined as the ratio of exposure area which canprovide an exposure amount that causes the matrix-like holes 77 not tobe resolved on the resist and causes the resist in the region to becompletely removed by the development, and which is also close to theratio of exposure area near the outside of the pattern forming region.

For instance, the ratio of the exposure area in FIG. 33 described nexthas a range within about 12% of the exposure area of the pattern formingregion. A range of the ratio of the exposure area in FIG. 34 is within36% of the exposure area of the pattern forming region. But, as shown inFIG. 37, different exposure amounts can be employed in the patternregions in FIGS. 33 and 34.

Here, respective examples in size of the island-like pattern 74 in thepattern forming region and the plurality of holes 77 outside the patternforming region are illustrated in FIGS. 33 to 35.

Dimensions shown in these Figures are mere one examples. Thesedimensions are reduced to, for example, 1/100 times on the resist.

As shown in FIG. 33, two island-like patterns 74 are formed in therectangular region of 100 μm×100 μm and opening region forming regions79, each having a width of 5 μm, are formed around two island-likepatterns 74. As shown in FIG. 34, one hundred holes 77 surrounded byframes 78 are formed in a matrix fashion in a rectangular region of 100μm×100 μm. As shown in FIG. 35, the hole 77 has a rectangular shape of 6μm×6 μm, and the crossbeam 78 is formed along the peripheral region ofthe hole 77 in a range of 10 μm×10 μm. Although the crossbeam 78 has awidth of 4 μm between the holes 77, the crossbeam 78 having such sizecannot be resolved on the positive type resist.

In the meanwhile, a relationship between regions of the opening regionsof the exposure mask and the current amount of the electron beam isshown like FIG. 36. Note that an amount of current flowing through theopening regions increases as the areas of the opening regions areincreased. Assume that an amount of current flowing through the openingregions is set to 1 under the condition where the crossbeams are notformed around the openings (full opening state), it can be seen that theamount of current passing through the hole 77 is reduced to 1/3 to 1/4if the plurality of holes 77 are formed by dividing the opening regionsby the crossbeams 78 shown in FIG. 34. For this purpose, an exposuretime required for one electron beam shot must be set to be longer thanthat of the full opening state.

Now, it should be noted that, although, as shown in FIG. 32, theisland-like pattern 74 and the matrix-like holes 77 are formed in oneexposure mask in the above description, they can be formed respectivelyin separate exposure masks.

Using the exposure mask described above, the positive type resist isexposed. If the ratio of the exposure area of the pattern non-formingregion 73 is different from that of the pattern forming region near thepattern non-forming region 73, these exposure amounts are made to bedifferent, so that the proximity effect can be corrected. The exposureamounts are set to values determined by the experiments performed by thepresent inventor.

For example, as shown in FIG. 37, a dose amount of the charged particlesinside the region spaced apart from the edge of the pattern formingregion 80 by 4 μm is set to 7.3 μC/cm². A dose amount of the chargedparticles in the region near the more central portion is also set to 7.8μC/cm². In addition, the charged particles are irradiated in an region81 outside of the pattern forming region 80 so as to obtain the doseamount of 6.8 μC/cm².

When the positive type resist is exposed with such dose amount by usingthe exposure mask having patterns in FIG. 32 and is over-exposed, aresult shown in a microphotograph of FIG. 38 has been derived. Theexposure apparatus shown in FIG. 15 has been employed to exposure theresist. In FIG. 18, black portions show the resist patterns.

When the island-like patterns near the outside of the pattern formingregion is enlarged and watched on the microphotograph, a result shown inFIG. 39A can be derived, i.e., the island-like patterns, each having agood shape, can be derived. Furthermore, when the island-like patternsnear the central portion of the pattern forming region is enlarged andwatched by the microphotograph, a result shown in FIG. 39B can bederived. At this time, a line width of the electron beam is set to beabout 0.13 μm.

On the contrary, upon exposing the region 81 outside the pattern formingregion 80, a result shown in a microphotograph of FIG. 40 can be derivedif the exposure mask without the matrix-like holes 77 shown in FIG. 34is employed, and the exposure amount is not changed. According to thismicrophotograph, it can be found that part of the island-like patternsnear the outside of the pattern forming region is eliminated, or theisland-like patterns are narrowed as shown in the enlargedmicrophotograph in FIG. 41A. In addition, a tendency to a narrow patternis not caused in the island-like patterns in the central region of thepattern forming region, but, as shown in FIG. 41B, there exists theisland-like patterns having narrow distances therebetween and theisland-like patterns coupled to adjacent island-like patterns.

With the above, if a wide region of the resist is smeared away by theelectron beam, the exposure mask having the matrix-like holes as in thethird embodiment, if employed, can reduce the coulomb interaction andsuppress bad influences on the patterns significantly. Moreover, sincethe exposure mask can be formed by simply opening holes 77 in thesilicon substrate, it can be formed simply in the process and strong. Inthe exposure mask, if sizes and locations of the holes 77 are determinedto make an exposure amount per unit area uniform, the reflected electronintensity can be neglected. When the exposure mask having thematrix-like holes is employed, the exposure of the resist can beattained sufficiently by one electron beam shot and therefore thethroughput thereof is scarcely reduced.

Although the exposure of the positive-type resist has been explained inthe third embodiment, the present embodiment can also be applied to thenegative type resist in the meaning of suppressing thecross-interaction.

Incidentally, dimensions of the opening regions and the patterns in theabove embodiments are not restricted to these dimensions. Also thepatterns of the exposure masks are reduced on the resist and transferredthereon.

(Fourth Embodiment)

In a fourth embodiment of the present invention, there will be explainedanother exposure method of suppressing degradation in pattern precisionproduced by the proximity effect.

The above degradation in pattern precision due to the proximity effectwill be not only caused by a shape and a size of each aperture pattern(i.e., transmission hole) formed in the block-mask, but also determineddepending on whether or not another pattern surrounding the periphery ofthe aperture pattern exists. The block-mask will be referred to as amask hereinafter.

As shown in FIG. 43, for instance, if an insulating layer 202 such as anpatterning object on a semiconductor wafer 201 is patterned, a EB resistfilm 203 is first applied to the insulating layer 202, then a chargedparticle beam (referred to as "electron beam" hereinafter) is irradiatedonto the resist film 203. The electron beam is then scattered in theresist film 203, insulating layer 202, and the semiconductor wafer 201,so that the proximity effect may be produced as illustrated in FIG. 43.

As also shown in FIG. 44, formed in a mask 210 for use in exposure are aplurality of block patterns, each of which has a plurality oftransmission holes (electron beam through holes) 211 to 214 therein. Byway of example, when the electron beam is irradiated onto such blockpatterns in terms of one shot projection, it passes through the first tofourth transmission holes 211 to 214, and is then reduced by anelectromagnetic lens so as to irradiate onto the resist film 203.

The first to fourth transmission holes 211 to 214 are respectivelyquadrangular patterns to have the same length in the y direction, whichare aligned in parallel at a distance. The first transmission hole 211is a square pattern having a largest area, while the second to fourthtransmission holes 212 to 214 are respectively rectangular patterns.

Curves 221 to 224 shown in FIG. 44 denote exposure amount on the resistfilm 203 provided by electron beams passing through respectivetransmission holes 211 to 214 if viewed from the I--I line. Asuperposition of these curves 221 to 224 results in a curve 220 shown inFIG. 44.

If the resist film 203 being exposed is developed, respective transferpatterns may be obtained as reduced patterns of the first to fourthtransmission holes 211 to 214. In fact, as shown in FIG. 44, onlyregions which are exposed in excess of the threshold value appear asfirst to third transfer patterns 231 to 233.

Referring to FIG. 44, the first to fourth transmission holes 211 to 213and the first to third transfer patterns 231 to 233 are depicted on thesame scale. Actually, sizes of the first to fourth transmission holes211 to 213 are however n time as large as those of the transferpatterns. Therefore, when being designed, exposure positions and sizesof the transfer patterns 231 to 233 depicted on the abscissa of thegraph are reduced 1/n times in contrast to the mask 210.

As can be seen from FIG. 44, because of the proximity effect, widths ofthe transfer patterns 232 corresponding respectively to the firsttransmission hole 211 having the largest area and the secondtransmission hole 212 adjacent to the hole 211 are expanded in the xdirection in comparison with those of desired transfer patterns. On theother hand, because of the proximity effect, widths of other transferpatterns 232 are contracted in the x direction in contrast to those ofdesired transfer patterns. Besides, transfer patterns corresponding tothe fourth transmission holes 214 are not formed. However, the lengthsof the first to third transfer patterns 231 to 233 in the y directionare derived as the substantially desired length.

Furthermore, as shown in FIG. 44, since a larger amount of scatteredelectrons are gathered to a side portion P1 of the first transmissionhole 211 from its surroundings in contrast to those gathered to a cornerportion P2 from its surroundings, the transfer pattern 231 is bulged. Asa result, the largest transfer pattern 231 and the neighboring transferpattern 232 come close to each other at their center portions of sides.

As for pattern deformation due to the proximity effect, there can becorrection methods (1) to (3) as follows:

(1) After the resist is exposed slightly employing an additional mask inwhich inverted patterns of light transmission regions and lightshielding regions are formed, it is exposed using the mask havingrequired regular patterns therein. This exposure method is called as aghost exposure method.

(2) The entirety of the exposure patterns is divided into a plurality ofrectangular regions, i.e., a plurality of blocks, then an exposureamount suited for each rectangular region is calculated, and then themagnitude of the exposure amount every rectangular region is adjusted soas to correct the proximity effect. As one method of adjusting theexposure amount, size of the blocks may be varied.

(3) Metal nets are pitched over the transmission holes, then an amountof the charged particles transmitted through the transmission holes ischanged by varying area and/or sizes of meshes of the metal net.

However, in the method (1), both the regular patterns and the invertedpatterns must be exposed. In the method (2), the size of the rectangularregions must be contracted. Throughput of exposure is thus loweredinevitably in both methods (1) and (2). In the method (3), since themetal net is frangible and broken down easily, it is hard to fabricatethe mask, so that much time is required to prepare the mask. In anyevent, since an appropriate exposure amount to correct the proximityeffect would not be uniform in one transmission hole, all theaforementioned methods (1) to (3) are difficult to meet the aboverequirement. Bulge of the transfer patterns cannot therefore beeliminated by the above methods (1) to (3). For this reason,miniaturization of the patterns would be restricted.

Subsequently, the fourth embodiment of the present invention will beexplained in more detail hereinafter.

FIRST EXAMPLE

Referring to FIGS. 45A and 45B, a mask forming method according to afirst example of the fourth embodiment will be explained.

This mask may be used in the electron beam exposure apparatus to exposethe resist in terms of block exposure. The block exposure is such anexposure method that the electron beam is shaped by the block patternsand then irradiated onto the resist.

A large number of block patterns to be used repeatedly are provided inthe mask. The electron beam is shaped by selected block pattern. Theblock pattern is thus reduced and projected by one shot electron beamonto the resist on the exposure object (e.g., semiconductor wafer).

As shown in FIG. 45A, first to fourth pattern regions 241 to 244 similarto desired patterns are arranged in a first design region 241 whereinblock patterns are to be formed. Outer shapes of the first to fourthpattern regions 241 to 244 are identical to those of the transmissionholes 211 to 214 shown in FIG. 44.

Next, as also shown in FIG. 45A, the first to fourth pattern regions 241to 244 are divided respectively into plural divisional rectangularregions 245, each having the same size.

As shown in FIG. 45B, the rectangular regions 246 of the same size arethen arranged one by one in respective divisional rectangular regions245. Where it is assumed that a ratio of the area (S2) of therectangular regions 246 to the area (S1) of the divisional rectangularregions 245 is k (k=S2/S1), and a proper exposure amount, i.e.,threshold exposure amount in the case of FIG. 44 is E0.

Depending upon design data such as sizes, positions etc. of the first tofourth pattern regions 241 to 244, the divisional rectangular regions245, and the rectangular regions 246 in such design region 240, the maskusing the rectangular regions 246 as the transmission holes has beenformed. If the electron beam is irradiated onto the transmission holes246 of the above mask under the exposure amount E=Eo/k, transferpatterns can be obtained which are substantially identical to thetransfer patterns 231 to 233 shown in FIG. 44 not subjected to theproximity correction. This fact has been confirmed experimentally. Thearea S_(o) of the transmission hole is not expressed by the dimension onthe wafer, but the dimension on the mask.

In this case, because of diffraction of electron beams transmittedthrough the transmission holes and the proximity effect caused in theresist film, the charged particles are irradiated onto the resist filmportions corresponding to regions between neighboring divisionalrectangular regions 245.

In order to reduce the influence of exposure due to the proximityeffect, in the resist film, an exposure amount through one rectangularregion 246 (transmission hole) increased by scattered electrons throughfrom other rectangular regions (trans-parent holes) has to becalculated. If, with increase of the increased exposure amount, the sizeof the rectangular regions 246 (transmission holes) is made smaller thanthe area S_(o) to thus correct the exposure amount, bulge and reductionof the first to fourth pattern regions 241 to 244 can be suppressed.However, even if scattered electrons generated by the proximity effectetc. remain therein, the area S_(o) of the rectangular region 246(transmission hole) providing a minimum exposure amount is maintained asit is, and therefore the size of the rectangular region 246 is notchanged.

It will in turn be explained with reference to FIG. 46 to form an actualmask employing the rectangular regions 246 having the corrected size asthe transmission hole.

As shown in FIG. 46, the transmission holes 256 of the size obtainedafter the above design data of the rectangular region 246 beingcorrected are formed on the silicon substrate by the photolithographytechnique, for example, so as to fabricate the mask 250. For purposes ofsimple illustration, only block patterns onto which one shot electronbeams are irradiated are depicted in FIG. 46 although a number of blockpatterns are in fact formed in the mask 250. Block patterns includerespectively the first to fourth pattern regions 241 to 244.

In the mask 250 in FIG. 46, the charged particles are passed through thetransmission holes a to g in regions formed along the line II--II andthen irradiated onto the exposure object so as to exhibit exposure(charged particle amount) distribution indicated by the chain lines inFIG. 46. If a superposition of these chain lines result in the curve 60indicated by the solid line. If the resist being subjected to theexposure is developed, transfer patterns 271 to 274 can be derived, asshown in FIG. 46, to retain desired patterns in which the proximityeffect has already been corrected.

Since the proximity effect to be generated in respective portions of thefirst to fourth pattern regions 241 to 244 may be corrected by sizeadjustment of the transmission hole 256, sufficient correction of theproximity effect can be provided. As a result, bulge and thinness shownin FIG. 44 can be reduced.

Since both the regular pattern mask and the inverted pattern mask asmentioned above are not required or since the block patterns are notcontracted to correct the proximity effect, reduction in throughput ofthe exposure process can be prevented. Furthermore, the mask structureon which the metal net is put up can be eliminated, easy maskfabrication can be achieved.

SECOND EXAMPLE

FIG. 47 shows a transfer pattern to be formed on the positive typeresist, which consists of block regions 280 divided by the chain lineand block masks corresponding to B1 to B8. In FIG. 47, charged particlesare irradiated onto the hatched region, and an exposure amount to beremoved by the developing liquid is required therein.

If such transfer patterns are formed, left portions 281 to 284, A1 to A5result in on the resist. When the light shielding portions correspondingto such left portions 281 to 284, A1 to A5 are formed in theconventional mask (not shown), such light shielding portions are notformed as the light shielding portions, i.e., they are omitted since thetransmission holes are formed around such light shielding portions.

FIG. 48 shows the block region 280 in FIG. 47 in an enlarged fashion. Ifthe structure illustrated in the first example is adopted to form theblock region 280 on the mask, the mask will be obtained wherein hatchedportions shown in FIG. 49 serve as the transmission holes.

According to the mask, light shielding portions corresponding to theleft portions 281 to 284 are supported by their peripheral portions.Besides, throughput can be enhanced in contrast to the exposure methodrecited in Patent Application Publication (KOKOKU) 63-11657 wherein twopatterns are exposed separately.

Even if plural transfer patterns are formed in one block of the resistto have the same shape, different patterns are formed on the block maskcorrespondingly to the plural transfer patterns based on respectivesurrounding pattern arrangements, etc. This fact would render design ofthe block mask more troublesome than the conventional one. However, theblock mask may be utilized many times in exposure, so it has highavailability.

THIRD EXAMPLE

Since all the exposure patterns of the mask can in general bepartitioned into rectangular patterns, it becomes important how to formordinarily the transmission holes which are to be formed in therectangular patterns. A design method of sizes and locations of thetransmission holes and a forming method of the transmission holes willbe explained with reference to FIGS. 50 to 53 hereinbelow.

As shown in FIG. 50, first a rectangular pattern 290 is partitioned intoplural portions on the basis of standard rectangular patterns P41 indesign stage. The electronbeam through the rectangular pattern 290 isirradiated onto a resist film.

It is assumed that a length of the rectangular pattern 290 in the xdirection is L_(x), and a length of the same in the y direction isL_(y). And, it is assumed that a length of the standard rectangularpattern P41 in the x direction (rectangular divisional standard length)is M_(x), and a length of the same in the y direction (rectangulardivisional standard length) is M_(y).

The length of M_(x) and M_(y) would be decided, for example, as follows.

First several lengths are selected as M_(x) and M_(y) respectivelywithin 0.1 to 0.3 μm. This range of 0.1 to 0.3 μm being used is selectedbecause it is believed that the optimum value will be obtained withinthe range of 0.2 μm±0.1 μm, where a line width of 0.2 μm is a minimumone to form patterns.

Then as for respective lengths, dimensions of the transparent holes aredecided according to design process described according to designprocess described hereinafter. After the transparent holes areimplemented in the mask, electron beam exposure is then carried out. Onthe basis of the results of such experiments or further calculations,the length of M_(x) and M_(y) may be finally decided which allow resultpatterns to have highest precision.

As the result that the rectangular pattern 290 have been partitionedbased on the standard rectangular patterns P41, smaller area portionsthan the standard rectangular pattern P41 sometimes remain. In thisevent, reduced patterns wherein a length of the standard pattern in thex direction is N_(x) and a length of the same in the y direction isN_(y) are calculated according to the following two equations to use assecondary standard patterns 295.

    N.sub.x =L.sub.x / L.sub.x /M.sub.x !                      . . . (40)

    N.sub.y =L.sub.y / L.sub.y /M.sub.y !                      . . . (41)

In the above equations, ! means an integral number obtained by raisingof less than unit, i.e., raising of fraction less than decimal point. Asize of the secondary standard pattern 295 is so determined that, ifthis secondary standard pattern 295 is used as the transmission hole,the resist exposed by the charged particles passing through thetransmission holes can not be resolved due to the proximity effect as aboundary line pattern of the transmission holes.

Subsequently, as shown in FIG. 51, the rectangular pattern 290 ispartitioned on the basis of the secondary standard patterns 295. Morespecifically, the secondary standard patterns 295 of integral number arespread all over the rectangular pattern 290.

Then, as shown in FIG. 51, divisional rectangular patterns 296 aresuperposed on respective secondary rectangular standard patterns 295.The divisional rectangular pattern 296 has a length S_(x) =N_(x) -2d_(x)in the x direction and a length S_(y) =N_(y) -2d_(y) in the y direction.Where 2d_(x) denotes a distance between adjacent divisional rectangularpatterns 296 in the x direction, and 2d_(y) denotes a distance betweenadjacent divisional rectangular patterns 296 in the y direction. Formedbetween edges of the divisional rectangular patterns 296 and edges ofthe secondary standard patterns 295 are rectangular frame-like areaseach of which has a width d_(x) in the x direction and a width d_(y) inthe y direction. These rectangular frame-like areas serve as crossbeamsin the actual mask.

A size of the divisional rectangular pattern 296 may be determined inthe following manner. In other words, in case the divisional rectangularpatterns 296 are used as the transmission holes, the charged particlespassing through the transmission holes to irradiate onto the resist maybe spread over the areas, which can be resolved to have the same size asreduced dimension of the secondary standard pattern 295, because ofdiffraction of the charged particles and influence of the proximityeffect. On the other hand, two distances 2d_(x), 2d_(y) between thedivisional rectangular patterns 296 are determined to meet the followingtwo conditions.

(1) In the event that the divisional rectangular patterns 296 are usedas the transmission holes, the resist is exposed to the electron beamspassing through the transmission holes not to transfer the frame-likeareas thereon because of diffraction of the charged particles andinfluence of the proximity effect. Consequently, continuous transferpatterns can be formed on the resist.

(2) Sufficient mechanical strength of the mask wherein the divisionalrectangular patterns 296 are employed as the transmission holes can beassured by the frames formed in the frame-like regions.

Moreover, d_(x) and d_(y) become too small if N_(x) and N_(y) are settoo small, while the proximity effect may be corrected insufficiently ifN_(x) and N_(y) are set too large. N_(x) and N_(y) must thus be setwithin an appropriate range.

Basically the electron beams should be irradiated onto the entirety ofthe secondary standard patterns 295. However, since the electron beamactually pass through the divisional rectangular patterns 296 in thethird example, a total amount of the charged particles to be irradiatedonto the resist may be reduced less than a desired amount E0. Thus, notto reduce an irradiation amount of the charged particles, usualirradiation time T₀ for one shot must be corrected to T according to thefollowing equation.

    T=T.sub.0 ·(N.sub.x /S.sub.x)·(N.sub.y /S.sub.y). . . (42)

Dimensions d_(x), d_(y), S_(x), S_(y) are further modified everysecondary standard pattern 295 as shown in the following. The size ofthe aforementioned rectangular pattern 290 is supposed to form patternson the resist on a reduced scale of 1/100, for example. In this case, asshown in FIG. 52, if the entire area of the rectangular pattern 290 isused as the transmission hole, the patterns formed on the resist arebulged with the proximity effect etc. when such transmission hole is toolarge.

Therefore, in further design stage, after the divisional rectangularpatterns 296 are arranged one by one in respective secondary standardpatterns 295, size of the divisional rectangular patterns 296 may bemodified by calculating a correction amount of the proximity effect asfollows:

When the electron beam is irradiated on a point O, the charged particlesare spread over a neighboring area of the point O because of theproximity effect to thus form rotationally symmetric charged particle(exposure amount (dosage)) distribution. If a distance from the point Ois r, dosage f(r) on a point remote from the point O by the distance rcan be derived from the following equation.

    f(r)=a·exp(-(r/b).sup.2)+c·exp(-(r/d).sup.2). . . (43)

In this equation (43), a first term expresses forward scattering due tothe proximity effect, and a second term expresses back scattering due tothe proximity effect. Constants a to d can be derived from theexperiment. The equation (43) has already been known in the art.

Here r denotes a distance on the resist and the distance becomes n timeson the mask. But this n may be applied if the patterns in the mask isexposed on the reduced scale of 1/n.

According to the equation (43) of f(r), the charged particledistribution having its peak on the point O can be formed. The distancer is given as R where the peak value of the charged particledistribution is attenuated to a rate p. A magnitude of the chargedparticle amount attenuated from the peak value to a rate p is anexposure threshold, for example.

In this event, the following relation may be formulated if the dosage atthe distance R from the point O is assumed as f(R).

    f(R)/f(0)=p (selected value)                               . . . (44)

The value p is a smaller value than a unit such as 1/8, for instance. Inaddition, f(R) may be set to the charged particle amount providing theexposure threshold value, for example.

Next, as shown in FIG. 52, virtual lines T_(x), T_(y) are drawn at thedistance R from the intersection point of diagonal lines of therectangular pattern 290 in the x, y directions. Four rectangular regions297 are arranged which are adjacent to the intersection point of thevirtual lines T_(x), T_(y) and have the virtual lines T_(x), T_(y) asrespective two sides. Other rectangular regions 297 having the same sizeare then arranged adjacently to the circumference of the rectangularregion 297. It may be evident that other patterns (not shown) arearranged around the rectangular pattern 290.

Now an occupied area ratio of the rectangular pattern 290 to therectangular region 297 is set at λ. The area ratio λ of the rectangularregion to sixteen surrounding rectangular regions are then calculated.In FIG. 52, the area ratio λ is indicated by the numeral put inparentheses. For example, it may be seen that, in the area labelednumeral 0 in the parentheses, no transmission hole is formed.

With reference to FIG. 52, an exposure correction coefficient Q is thencalculated to correct S_(x) and S_(y) shown in FIG. 51. The exposurecorrection coefficient Q can be derived from the following equation.

    Q=1/(1+C·η)                                   . . . (45)

The correction coefficient Q is less than or equal to a unit. Thesmaller the value of the exposure correction coefficient Q, the more thecorrection amount.

For instance, the value of C would be determined in the followingmanner. First several values of C must be selected within the range of 0to 1. Then as to respective values, dimensions of the transparent holesare decided according to design process discussed hereinafter. Thetransparent holes are then implemented in the mask to effect theelectron beam exposure. The value of C required for permitting resultantpatterns to be achieved with highest precision can be determinedaccording to the results of either experiments or calculations.

Where C is a constant obtained according to the experiment, and lessthan or equal to a unit. The η denotes the charged particle amount whichis added to i-th rectangular region 297 due to the proximity effect. Thecharged particle amount may be obtained due to both the proximity effectcaused by the i-th rectangular region 297 per se and the proximityeffect caused by m neighboring rectangular regions 297.

The i-th η_(i) can be calculated by the following equation.

    η.sub.i =Σλ.sub.i ·T.sub.x ·T.sub.y ·f(r.sub.i)                                      . . . (46)

Where a symbol Σ denotes a total sum of i=0 to m. In FIG. 52, i=0 to 8.Further, λ_(i) denotes the area ratio λ in the i-th rectangular region297, and r_(i) denotes the distance between the center of gravity of thei-th rectangular region 297 and the center of gravity of the neighboringrectangular region 297. Now the distance r₀ is 0.

For example, the following is the charged particle amount due to theproximity effect caused by the 0-th (i=0) rectangular region 297 in FIG.52.

Here, if T_(x), T_(y) are 1 μm respectively, distances r_(i), r₃₁, r₅and r₇ from the center of gravity of the 0-th (i-th) rectangular region297 to the 1-st, 3-rd, 5-th, and 7-th (i-th) rectangular region are "1"respectively. Distances r₂, r₄, r₆ and r₈ from the center of gravity ofthe 0-th (i-th) rectangular region 297 to the 2-nd, 4-th, 6-th, and 8-th(i-th) rectangular region are "√2" respectively. In addition, the arearatios λ₁, λ₆, λ₇ of the 1-st, 2-nd, 3-rd rectangular regions 297 are0.8 respectively, the area ratio λ₄ of the 4-th rectangular region 297is 0.2, the area ratios λ₅, λ₆, λ₇ of the 5-th, 6-th, 7-th rectangularregions 297 are 0 respectively, and the area ratio λ₈ of the 8-threctangular region 297 is 0.4. That is, ##EQU2##

By calculating all η_(i) in the same manner as above, Q_(i) in the i-threctangular region 297 can be derived. Among plural Q_(i) 's in theblock patterns within the one shot exposure range, the largest Q_(i) isdefined as Q_(max).

In the meanwhile, since the divisional rectangular pattern 296 of S_(x)·S_(y) has already been surrounded by the frame-like regions, S_(x)·S_(y) is not corrected in the region Q_(max).

Based on the exposure correction coefficient Q_(i) obtained as above,the dimension S_(x) of the divisional rectangular pattern 296 iscorrected to U_(x) and the dimension S_(y) of the same is corrected toU_(y). The following is the equation for correction. Here U_(xi) denotesthe i-th dimension in the x direction while U_(yi) denotes the i-thdimension in the y direction. ##EQU3##

Since Q_(i), Q_(max) are the correction coefficients as for areas,square roots of the correction coefficients Q_(i), Q_(max) are used inthese equations to coincide their dimension to each other.

From these equations (47), (48), the following equation can be obtained.

    U.sub.xi ·U.sub.yi (Q.sub.i /Q.sub.max)·S.sub.x ·S.sub.y                                         . . . (49)

Besides, the equation (49) can be expressed from the above relation ofQ_(i) =1/(1+C·η_(i)) as follows:

    U.sub.xi U.sub.yi (1+C·η.sub.i)=S.sub.x ·S.sub.y /Q.sub.max =constant                                      . . . (50)

It can be seen from this relation that the equation representing theabove Q_(i), η_(i), U_(xi) U_(yi) is a linear approximate correctionexpression with respect to the charged particle amount due to theproximity effect.

With the above, the divisional rectangular patterns 296 included in therectangular regions 297 may corrected by the same amount by the sametechnique. As shown in FIG. 53, the corrected divisional rectangularpatterns 296 are realized as the transmission holes 298.

To calculate η_(i) exactly, surface integral value of f(r) may besubstituted for f(r) in the above equation (46).

Now the following shows a concrete example of the mask. The mask may beconstituted by forming the transmission holes in the patterning regionhaving a 20 μm thickness of the silicon substrate of a 600 μm thickness.M_(x) and M_(y) are 8 μm respectively; 2d_(x) and 2d_(y) are 2 μmrespectively; T_(x) and T_(y) are 1 to 4 μm respectively; and C is 0.5to 1.0 μm.

Note that various variations are also included in the present invention.

It would be understood that, although the rectangular pattern 290 ispartitioned into four rectangular regions 297 in the above explanation,any plural regions are available. In FIG. 52, in order to takediffraction of the charged particles and the proximity effect caused insixteen outer regions as well as eight regions around the rectangularregion 297 into consideration, the exposure correction coefficient Q maybe calculated by further reducing the rectangular region 297.

In addition, it should be noted that the above Q or η may be correctedbased on actually obtained transfer patterns.

Moreover, the case where the size is corrected by reducing thedivisional rectangular pattern has been explained in the first to thirdexamples, but on the contrary the same result as above may be obtainedto correct the size by magnifying the divisional rectangular pattern.

Besides, the size of the divisional rectangular patterns 296 may not bealways identical to each other. For this reason, the divisionalrectangular pattern having the larger size than those of peripheralpatterns may be placed at the center of the rectangular pattern 290.

What is claimed is:
 1. An electron beam exposure mask used for formingpatterns by one electron beam shot, comprising:electron beam shieldingpatterns for shielding charged particles; and a plurality of openingregions disposed along peripheral edges of said electron beam shieldingpatterns and divided by bridging portions, wherein each of said bridgingportions has a length not to be resolved in an EB resist by scatteringof said charged particles in an alignment direction of said openingregions, and each of said opening regions has a width formed narrowly bya bulge amount caused by said scattering of said charged particles inthe direction intersecting said alignment direction.
 2. An electron beamexposure mask according to claim 1, wherein said plurality of openingregions are disposed in plural directions, andone second opening regionisolated by said bridging portion is disposed in each intersectingregion of the plural directions, said second opening regions beingformed smaller than said opening regions disposed in remaining regions.3. An electron beam exposure mask according to claim 1, wherein saidelectron beam shielding patterns are formed as island-like orpeninsula-like patterns surrounded by said opening regions.
 4. Anelectron beam exposure mask according to claim 1, wherein a length ofeach of said opening regions in an alignment direction is 1.5 or moretimes as long as a length of each of said bridging portions inlongitudinal direction.
 5. An electron beam exposure mask according toclaim 1, wherein said plurality of opening regions have substantiallythe same exposure region in a range of one electron beam shot.
 6. Anelectron beam exposure mask according to claim 1, wherein said pluralityof opening regions are disposed in series in one direction.
 7. Anelectron beam exposure mask according to claim 1, wherein said pluralityof opening regions are formed on a silicon substrate.
 8. An electronbeam exposure mask comprising:a plurality of electron beam transmittingholes having aperture regions which become large in one direction in arange where charged particles can be irradiated by one shot; and aplurality of electron beam shielding regions for dividing said exposuremask into said plurality of electron beam transmitting holes and eachhaving a size not to be resolved.
 9. An electron beam exposure maskaccording to claim 8, wherein said electron beam transmitting holes areformed on a silicon substrate.
 10. A method of manufacturing an electronbeam exposure mask comprising the steps of:calculating a reflectionstrength distribution of charged particles in a resist layer, thecharged particles having been transmitted through an exposure mask;finding regions in which the reflection intensity decreases as adistance from a center of an opening in said exposure mask increases;and making a plurality of openings in a mask substrate, each area ofsaid openings increases as the reflection intensity in said regionsdecreases.
 11. A method of manufacturing an electron beam exposure maskcomprising the steps of:dividing a region including patterns to beexposed on a resist into a plurality of rectangular regions which aresmaller than said patterns; estimating reflected electron intensity,caused when said patterns are exposed on said resist, on every vertex ofsaid rectangular regions; selecting, as the first region, saidrectangular region wherein two lines connecting vertexes of saidreflected electron intensity having similar electron intensity of fourvertexes become parallel by comparing said reflected electronintensities on four vertexes of each of said rectangular regions;setting, as the first change rate, a change rate of the reflectedelectron intensity between the first vertex and the second vertex in thedirection perpendicular to said two lines in said first region;selecting, as the second region, said rectangular region wherein saidreflected electron intensities are divided into three or four types bycomparing said reflected electron intensities on four vertexes in saidrectangular region; setting, as the second change rate, a change rate ofsaid reflected electron intensity between the third vertex and thefourth vertex where the maximum value and the minimum value of saidreflected electron intensity are provided, in four vertexes in saidsecond region; setting said first vertex having a large reflectedelectron intensity as the smallest opening area and said second vertexas the largest opening area in said first region, and forming aplurality of electron beam transmitting holes, opening areas of whichchange in proportion to said first change rate, between said firstvertex and said second vertex in said first region; and setting saidthird vertex having a large reflected electron intensity as the smallestopening area and said fourth vertex as the largest opening area in saidsecond region, and forming a plurality of electron beam transmittingholes, opening areas of which change in proportion to said second changerate, between said third vertex and said fourth vertex in said secondregion.
 12. A method of forming a charged particle beam mask, comprisingthe steps of:partitioning one pattern into a plurality of patterns basedon a first standard rectangle; arranging second standard rectangles inrespective first standard rectangles; calculating an amount of chargedparticles as for said first standard rectangles due to proximity effectin exposure regions on which said first standard rectangles are reducedand exposed; correcting size of said second standard rectangles so thatsaid second standard rectangles in said first standard rectangles havingsmall amount of said charged particles are reduced smaller than thesecond standard rectangles in said first standard rectangles havinglarge amount of said charged particles; and forming transmission holesby transferring respective second standard rectangles onto said masksubstrate after respective areas of said second standard rectanglesbeing corrected.
 13. A method of forming a charged particle beam mask,comprising the steps of:partitioning one pattern into a plurality ofpatterns based on a first standard rectangle; arranging second standardrectangles in respective first standard rectangles; partitioning aregion including said one pattern and its neighboring region into aplurality of patterns based on a third standard rectangle larger thansaid first standard rectangle; calculating an amount of chargedparticles as for said third standard rectangles due to proximity effectcaused in said third standard rectangles per se and proximity effectcaused in areas surrounding said third standard rectangles, in exposureregions on which said third standard rectangles are reduced and exposed;correcting size of said second standard rectangles so that said secondstandard rectangles in said third standard rectangles having smallamount of said charged particles are reduced smaller than the secondstandard rectangles in said third standard rectangles having largeamount of said charged particles; and forming transmission holes bytransferring respective second standard rectangles onto said masksubstrate after respective areas of said second standard rectanglesbeing corrected.
 14. A method of forming a charged particle beam maskaccording to claim 13, wherein said one pattern can be partitionedregularly based on integral number of said first standard rectangle. 15.A method of forming a charged particle beam mask according to claim 13,wherein a plurality of said first standard rectangles have the same sizerespectively, and a plurality of said second standard rectangles havethe same size respectively.
 16. A method of forming a charged particlebeam mask according to claim 13, wherein if a length of said secondstandard rectangle in an x direction is set to S_(x) and a length ofsaid second standard rectangle in a y direction perpendicular to said xdirection is set to S_(y), and an amount of charged particles due toproximity effect caused in said third standard rectangles and proximityeffect caused in their neighboring regions is set to η, and a correctioncoefficient as for said third standard rectangles shown by the followingequation including a constant C is set to Q,

    Q.sub.i =1(1+Cη.sub.i)

and a maximum value of said Q's in a plurality of said third standardrectangles included within one shot charged particle beam region is setto O_(max), then said S_(x) and S_(y) of said second standard rectanglemay be corrected to U_(x) and U_(y) respectively according to afollowing equation, ##EQU4##
 17. A method of manufacturing an electronbeam exposure mask, comprising the steps of:dividing entire exposureregions including pattern forming regions and non-pattern formingregions into a plurality of sections by setting one shot range of anelectron beam as one section; determining the ratio of exposure in saidnon-pattern forming regions located near said pattern forming regions sothat, when the ratio of exposure is defined as a value of dividing anelectron transmitting region by an entire area of said section persection, it becomes a close value to that of the peripheral area of saidpattern forming regions and said pattern forming regions do not resolveon a positive type resist after completing a development process for theresist.
 18. A method of manufacturing an electron beam exposure maskaccording to claim 17, wherein the step of said determining the ratio ofexposure in said non-pattern forming regions is performed by forming aplurality of electron beam transmitting holes having sizes and pitchesso that said holes do not resolve on a resist after completing adevelopment process for the resist and the ratio of exposure in saidnon-pattern forming regions located near said pattern forming regionsbecomes a close value to that of the peripheral area of said patternforming regions.
 19. A charged particle beam mask comprising:a masksubstrate; at least one pattern formed on a mask substrate; said patternhaving a first region having a relatively large amount of chargedparticles per area due to proximity effect and a second region having arelatively small amount of charged particles; and at least one firsttransmission hole formed in said first region and at least one secondtransmission hole formed in said second region; wherein said firstregion being adjacent to said second region interposed with a framehaving a width which is not affected by exposure to an electron beam;said amount of charged particles per area is determined by the electronbeam irradiated through transmission holes formed in said pattern andother pattern.